python多線程方法詳解

更新時間：2022年01月18日 09:28:48 作者：python_rser

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處理多個數(shù)據(jù)和多文件時，使用for循環(huán)的速度非常慢，此時需要用多線程來加速運行進度，常用的模塊為multiprocess和joblib，下面對兩種包我常用的方法進行說明。

1、模塊安裝

pip install multiprocessing
pip install joblib

2、以分塊計算NDVI為例

首先導入需要的包

import numpy as np
from osgeo import gdal
import time
from multiprocessing import cpu_count
from multiprocessing import Pool
from joblib import Parallel, delayed

定義GdalUtil類，以讀取遙感數(shù)據(jù)

class GdalUtil:
    def __init__(self):
        pass
    @staticmethod
    def read_file(raster_file, read_band=None):
        """讀取柵格數(shù)據(jù)"""
        # 注冊柵格驅動
        gdal.AllRegister()
        gdal.SetConfigOption('gdal_FILENAME_IS_UTF8', 'YES')
        # 打開輸入圖像
        dataset = gdal.Open(raster_file, gdal.GA_ReadOnly)
        if dataset == None:
            print('打開圖像{0} 失敗.\n', raster_file)
        # 列
        raster_width = dataset.RasterXSize
        # 行
        raster_height = dataset.RasterYSize
        # 讀取數(shù)據(jù)
        if read_band == None:
            data_array = dataset.ReadAsArray(0, 0, raster_width, raster_height)
        else:
            band = dataset.GetRasterBand(read_band)
            data_array = band.ReadAsArray(0, 0, raster_width, raster_height)
        return data_array
 
    @staticmethod
    def read_block_data(dataset, band_num, cols_read, rows_read, start_col=0, start_row=0):
        band = dataset.GetRasterBand(band_num)
        res_data = band.ReadAsArray(start_col, start_row, cols_read, rows_read)
        return res_data
 
    @staticmethod
    def get_raster_band(raster_path):
        # 注冊柵格驅動
        gdal.AllRegister()
        gdal.SetConfigOption('gdal_FILENAME_IS_UTF8', 'YES')
        # 打開輸入圖像
        dataset = gdal.Open(raster_path, gdal.GA_ReadOnly)
        if dataset == None:
            print('打開圖像{0} 失敗.\n', raster_path)
        raster_band = dataset.RasterCount
        return raster_band
 
    @staticmethod
    def get_file_size(raster_path):
        """獲取柵格仿射變換參數(shù)"""
        # 注冊柵格驅動
        gdal.AllRegister()
        gdal.SetConfigOption('gdal_FILENAME_IS_UTF8', 'YES')
 
        # 打開輸入圖像
        dataset = gdal.Open(raster_path, gdal.GA_ReadOnly)
        if dataset == None:
            print('打開圖像{0} 失敗.\n', raster_path)
        # 列
        raster_width = dataset.RasterXSize
        # 行
        raster_height = dataset.RasterYSize
        return raster_width, raster_height
 
    @staticmethod
    def get_file_geotransform(raster_path):
        """獲取柵格仿射變換參數(shù)"""
        # 注冊柵格驅動
        gdal.AllRegister()
        gdal.SetConfigOption('gdal_FILENAME_IS_UTF8', 'YES')
 
        # 打開輸入圖像
        dataset = gdal.Open(raster_path, gdal.GA_ReadOnly)
        if dataset == None:
            print('打開圖像{0} 失敗.\n', raster_path)
 
        # 獲取輸入圖像仿射變換參數(shù)
        input_geotransform = dataset.GetGeoTransform()
        return input_geotransform
 
    @staticmethod
    def get_file_proj(raster_path):
        """獲取柵格圖像空間參考"""
        # 注冊柵格驅動
        gdal.AllRegister()
        gdal.SetConfigOption('gdal_FILENAME_IS_UTF8', 'YES')
 
        # 打開輸入圖像
        dataset = gdal.Open(raster_path, gdal.GA_ReadOnly)
        if dataset == None:
            print('打開圖像{0} 失敗.\n', raster_path)
 
        # 獲取輸入圖像空間參考
        input_project = dataset.GetProjection()
        return input_project
 
    @staticmethod
    def write_file(dataset, geotransform, project, output_path, out_format='GTiff', eType=gdal.GDT_Float32):
        """寫入柵格"""
        if np.ndim(dataset) == 3:
            out_band, out_rows, out_cols = dataset.shape
        else:
            out_band = 1
            out_rows, out_cols = dataset.shape
 
        # 創(chuàng)建指定輸出格式的驅動
        out_driver = gdal.GetDriverByName(out_format)
        if out_driver == None:
            print('格式%s 不支持Creat()方法.\n', out_format)
            return
 
        out_dataset = out_driver.Create(output_path, xsize=out_cols,
                                        ysize=out_rows, bands=out_band,
                                        eType=eType)
        # 設置輸出圖像的仿射參數(shù)
        out_dataset.SetGeoTransform(geotransform)
 
        # 設置輸出圖像的投影參數(shù)
        out_dataset.SetProjection(project)
 
        # 寫出數(shù)據(jù)
        if out_band == 1:
            out_dataset.GetRasterBand(1).WriteArray(dataset)
        else:
            for i in range(out_band):
                out_dataset.GetRasterBand(i + 1).WriteArray(dataset[i])
        del out_dataset

定義計算NDVI的函數(shù)

def cal_ndvi(multi):
    '''
    計算高分NDVI
    :param multi:格式為列表，依次包含[遙感文件路徑，開始行號，開始列號，待讀的行數(shù)，待讀的列數(shù)]
    :return: NDVI數(shù)組
    '''
    input_file, start_col, start_row, cols_step, rows_step = multi
    dataset = gdal.Open(input_file, gdal.GA_ReadOnly)
    nir_data = GdalUtil.read_block_data(dataset, 4, cols_step, rows_step, start_col=start_col, start_row=start_row)
    red_data = GdalUtil.read_block_data(dataset, 3, cols_step, rows_step, start_col=start_col, start_row=start_row)
    ndvi = (nir_data - red_data) / (nir_data + red_data)
    ndvi[(ndvi > 1.5) | (ndvi < -1)] = 0
    return ndvi

定義主函數(shù)

if __name__ == "__main__":
    input_file = r'D:\originalData\GF1\namucuo2021.tif'
    output_file = r'D:\originalData\GF1\namucuo2021_ndvi.tif'
    method = 'joblib'
    # method = 'multiprocessing'
    # 獲取文件主要信息
    raster_cols, raster_rows = GdalUtil.get_file_size(input_file)
    geotransform = GdalUtil.get_file_geotransform(input_file)
    project = GdalUtil.get_file_proj(input_file)
    # 定義分塊大小
    rows_block_size = 50
    cols_block_size = 50
    multi = []
    for j in range(0, raster_rows, rows_block_size):
        for i in range(0, raster_cols, cols_block_size):
            if j + rows_block_size < raster_rows:
                rows_step = rows_block_size
            else:
                rows_step = raster_rows - j
            # 數(shù)據(jù)橫向步長
            if i + cols_block_size < raster_cols:
                cols_step = cols_block_size
            else:
                cols_step = raster_cols - i
            temp_multi = [input_file, i, j, cols_step, rows_step]
            multi.append(temp_multi)
 
    t1 = time.time()
    if method == 'multiprocessing':
        # multiprocessing方法
        pool = Pool(processes=cpu_count()-1)
        # 注意map函數(shù)中傳入的參數(shù)應該是可迭代對象，如list;返回值為list
        res = pool.map(cal_ndvi, multi)
        pool.close()
        pool.join()
    else:
        # joblib方法
        res = Parallel(n_jobs=-1)(delayed(cal_ndvi)(input_list) for input_list in multi)
 
    t2 = time.time()
    print("Total time:" + (t2 - t1).__str__())
 
    # 將multiprocessing中的結果提取出來，放回對應的矩陣位置中
    out_data = np.zeros([raster_rows, raster_cols], dtype='float')
    for result, input_multi in zip(res, multi):
        start_col = input_multi[1]
        start_row = input_multi[2]
        cols_step = input_multi[3]
        rows_step = input_multi[4]
        out_data[start_row:start_row + rows_step, start_col:start_col + cols_step] = result
 
    GdalUtil.write_file(out_data, geotransform, project, output_file)

雙重for循環(huán)時，兩層for循環(huán)都使用multiprocessing時會報錯，這時可以外層for循環(huán)使用joblib方法，內層for循環(huán)改為multiprocessing方法，不會報錯

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